This invention relates generally to postal weighing scales, and more particularly to scales that are operable both to weigh parcels and to weigh a number of small mailpieces in a differential weighing mode.
Postal weighing scales are well known. Generally the function of such a scale is to output a signal that indicates the weight of a mailpiece weighed on the scale. The weight signal may be output via a human-readable display, and a human operator may read the display and enter corresponding data into a postage meter so that the postage meter applies a proper amount of postage to the mailpiece.
In a more automated system, the scale may be connected to the postage meter via a signal path, and the scale may output a mailpiece weight signal directly to the postage meter. The meter then calculates and applies postage accordingly.
Some postal scales are arranged to weigh both relatively small mailpieces, such as standard no. 10 envelopes that enclose letters and weigh an ounce or two, as well as relatively large mailpieces, such as parcels weighing one to five pounds or more.
FIG. 1 is a simplified schematic side view of a conventional postal scale, which is generally indicated by reference numeral 100. The scale 100 includes a load cell 102 which is mounted via its fixed end 104 to a mounting pad 106. A weight distribution plate 108 is mounted on the free end 110 of the load cell 102. A weighing platter 112 is supported on the weight distribution plate 108. The platter 112 has an upper surface 114 on which mailpieces may be placed for weighing by the scale 100.
The housing of the scale 100 is indicated in phantom at 116. Other components of the scale, which are not shown or not separately shown, may include: a transducer (e.g., a strain gage) which may be a part of the load cell 102 which translates strain in the load cell 102 into a raw weight signal; signal conditioning and processing circuitry which processes the raw weight signal from the transducer to generate a meaningful weight signal, a data interface by which the scale 100 may communicate a weight signal to an external device such as a postage meter (not shown), and a user interface.
One known technique used for weighing mailpieces is referred to as “differential weighing”. In differential weighing, a batch of mailpieces (typically letters) is placed on the scale platter. Mailpieces are removed one-by-one from the scale platter and fed to a postage meter. As each mailpiece is removed from the scale platter, the scale detects, and determines the amount of, the reduction of the load on the platter. The detected reduction in load is, in effect, ascribed as the weight of the removed mailpiece and a corresponding weight signal is sent from the scale to the postage meter to allow the meter to calculate the appropriate amount of postage for the mailpiece. (Alternatively, the weight signal that corresponds to the reduction in load may be output on a display to the human operator.)
Differential weighing may be a more convenient and rapid way to weigh each mailpiece of a batch of mailpieces than placing the mailpieces on the scale platter one-by-one.
Typically when differential weighing is to be employed, a stack of envelopes may be placed on the scale platter. However, unless the stack is quite small, the stack may be unstable and likely to be upset after placement on the platter or as envelopes are removed from the stack, leading to inconvenience for the human operator and disruption of the differential weighing process.